By a previous application the inventors disclosed and claimed a set of alloys having a boron additive which made possible the achievement of a novel combination of strength and ductility in certain compositions. That application, Ser. No. 444,932 filed Nov. 29, 1982 now U.S. Pat. No. 4,478,791, was assigned to the same assignee as the subject application and is incorporated herein by reference.
It is pointed out in the prior application that in many systems composed of two or more metallic elements there may appear, under certain combinations of composition and treatment conditions, phases other than the primary solid solutions. Such other phases are commonly known as intermediate phases. Many intermediate phases are referred to by means of the Greek symbol such as .gamma. or .gamma.'. Also, they are preferred to by formula as, for example, Cu.sub.3 Al, CuZn and Mg.sub.2 Pb. The compositions of the intermediate phases which have simple approximate stoichiometric ratios of the elements may exist over a range of temperatures as well as compositions.
Occasionally, as in the case of Mg.sub.2 Pb, which occurs in the Mg-Pb system, a true stoichiometric compound, which compound is completely ordered, is found to occur. Where each of the elements of the compound is a metallic element, the intermediate compound itself is commonly called an intermetallic compound.
The intermediate phases and intermetallic compounds often exhibit properties entirely different from those of the component metals comprising the system. They also frequently have complex crystallographic structures. The lower order of crystal symmetry and fewer planes of dense atomic population of these complex crystallographic structures may be associated with certain differences in properties, e.g. greater hardness, lower ductility, lower electrical conductivity of the intermediate phases as compared to the properties of the primary solid solutions.
Although several intermediate intermetallic compounds with otherwise desirable properties, e.g. hardness, strength, stability and resistance to oxidation and corrosion at elevated temperatures, have been identified, their characteristic lack of ductility has posed formidable barriers to their use as structural materials. In fact, some of these materials are so friable that they have been prepared as solids in order that they may be broken up into powdered form for use in powder metallurgical processes for fabrication of articles.
A recent article appearing in the Japanese literature disclosed that the addition of trace amounts (0.05 to 0.1 wt. %) of boron to Ni.sub.3 Al polycrystalline material was successful in improving the ductility of the otherwise brittle and non-ductile intermetallic compound. See in this regard Journal of the Japan Institute of Metals, Vol. 43, page 358, published in 1979 by the authors Aoki and Izumi. Although the room temperature tensile strain to fracture of the Ni.sub.3 Al was improved by the boron addition to about 35%, as compared to about 3% for the Ni.sub.3 Al without boron, the room temperature yield strength remained at about 30 ksi. The Japanese article did not refer at all, however, to rapid solidification of the boron containing compositions which they studied.
By the method of the prior application for Ser. No. 444,932, filed Nov. 29, 1982 now U.S. Pat. No. 4,478,791, the addition of 0.01 to 2.5 at. % boron demonstrated further improvements where the alloy preparation included the step of rapid solidification. In particular, as it is brought out in this prior application, preferred properties are found in rapidly solidified compositions containing between 0.5 and 2.0% boron and an optimum combination of yield stress and strain to fracture is found in rapidly solidified compositions containing approximately 1.0% boron or less.
Surprisingly, it has now been found that further property improvements are possible in the alloy system of the gamma prime Ni.sub.3 Al intermediate phase where not only boron is present in the composition as a ternary element but in addition a metal is present as a quaternary ingredient of such compositions as a substituent metal.